Method for aligning a three-phase machine having soft start and three-phase machine

Abstract

In order to achieve the energy efficiency class IE4 defined in the IEC standard 60034, it is necessary to operate permanently excited synchronous machines directly on the mains. Because this is not readily possible, soft start devices may be considered as cost-efficient solutions. A method is described by which the initial rotor angle is defined, which can then be used by an encoderless start process. The fundamental concept is based on energizing in a defined direction. This is achieved in that solely two actuator phases are fired. A current space vector is thereby applied to the machine in a fixed direction and the machine is then aligned thereto. The successful alignment and a blocked motor can thus be recognized based on the profile of the stator current space vector.

What is claimed is: 
     
       1. A method for aligning an encoder-less three-phase machine having a soft start feature, comprising:
 a) firing antiparallel thyristors of two motor phases of the encoder-less three-phase machine in a first current direction with an initial large firing angle, 
 b) measuring an amplitude of phase currents, 
 c) decreasing the initial large firing angle until the measured amplitude of the phase currents approximates a predetermined maximum amplitude, with the decreased firing angle at the predetermined maximum amplitude representing an optimum firing angle of the antiparallel thyristors in the first current direction, 
 d) aligning the encoder-less three-phase machine with the two motor phases by using the previously determined optimum firing angle, 
 e) recording a course of the phase currents as a function of time as a reference course, and 
 f) energizing two different motor phases using the previously determined optimum firing angle and checking the alignment of the rotor in the second current direction by comparing a second course of the phase currents of the two different motor phases with the reference course over time and confirming that the encoder-less three-phase machine is aligned when a sum of error squares between the second course and the reference course is less than or equal to a first predetermined threshold value. 
 
     
     
       2. The method of  claim 1 , wherein a first phase current of the two motor phases has a current flow of Identical absolute value, but of opposite mathematical sign as a second phase current of the two motor phases. 
     
     
       3. The method of  claim 1 , further comprising firing the antiparallel thyristors of the two motor phases at least one more time until the measured amplitude is close to the predetermined maximum amplitude. 
     
     
       4. The method of  claim 1 , further comprising recording the reference course while the antiparallel thyristors of the two motor phases are fired and using the recorded reference course in subsequent measurements. 
     
     
       5. The method of  claim 1 , further comprising
 after step e) measuring the course of the phase currents at the optimum firing angle in a different second current direction, 
 comparing the course of the the phase currents in the different second current direction with the reference course, and 
 outputting an error message when a sum of error squares between the course in the second current direction and the reference course is greater than a predetermined second threshold value. 
 
     
     
       6. An encoder-less three-phase machine having a soft start feature, comprising:
 a rotor, 
 two antiparallel thyristors energizing two motor phases of the encoder-less three-phase machine, and 
 a thyristor controller configured to 
 determine an optimum firing angle of the two antiparallel thyristors in a first current direction to define a first alignment of a rotor of the encoder-less three-phase machine, with the optimum firing angle being determined by decreasing, with the thyristor controller, an initial large firing angle until a measured amplitude of phase currents approximates a predetermined maximum amplitude, 
 align the rotor with the first alignment by using only the two motor phases of the encoder-less three-phase machine and the previously determined optimum firing angle, 
 record a course of the phase currents as a reference course, 
 energize two different motor phases using the previously determined optimum firing angle, and 
 checking the alignment of the rotor in the second current direction by comparing a second course of the current space vector in the two different motor phases with the reference course over time and confirming that the encoder-less three-phase machine is aligned when a sum of error squares between the second course and the reference course is less than or equal to a first redetermined threshold value.